Typical adhesives generally are liquid or semi-liquid that bond similar or dissimilar materials together by curing through evaporation of solvents and/or chemical reaction of one or more constituents. While liquid or semi-liquid adhesives are useful in many joining applications, a disadvantage of liquid or semi-liquid adhesives is that it takes a long time to cure the adhesives, i.e., the joining step is not immediate. Furthermore, liquid or semi-liquid adhesives often give off volatile organic compounds (VOC) that are toxic, have limited pot life, and/or have limited shelf life. Liquid adhesives are difficult to apply in a uniform manner, often showing shrinkage. From a process and application viewpoint, heat and pressure, jigs and fixtures for the joint geometry, rigid process control, inspections, and training are often required. Other issues include variable viscosity, evaporation, and/or substrate contamination.
Various typical dry adhesives have been developed to overcome the disadvantages of liquid adhesives. For example, pressure sensitive adhesive (PSA) single-coated tape products are examples of dry adhesive products consisting of a backing material, primer, adhesive, and release agent or liner. In general, pressure sensitive adhesive products require the adhesive to be tacky and easily deformable, i.e., soft, for adhesion to another surface, i.e. adherend. There are other adhesion considerations such as PSA chemistry, surface tension, and surface preparation that are considered to appropriately matching of PSA to various substances. The negative issues associated with PSAs include dwell time to gain full adhesive strength while under pressure, mismatch of surfaces, surface preparation time, lower adhesive strength, and limited shelf and working life.
Other typical dry adhesive products are hot melt adhesive products. Hot melt adhesive products are typically thermoplastic copolymers tackified with resins. Hot melt adhesive products have limited use because high temperature is required to melt the adhesive to join the desired surfaces. The hot melt adhesive is typically heated above its melting point to a molten state and rapidly solidifies to form the bond between the desired surfaces. There are many disadvantages to hot melt adhesives including potential injury to persons applying the adhesive, time to heat the adhesive, heat damage to the substrate, and limited range of properties.
Although Velcro® is a mechanical fastener using hook-and-loop and is not an adhesive, Velcro® is employed as a dry fastener to join together similar and/or dissimilar surfaces. Velcro® typically consists of two layers. A first layer consists of a first backing covered with tiny hooks, and a second layer consists of a second backing covered with tiny loops. When the hook side is pressed together with the loop side, the loops catch the hooks and hold the first backing together with the second backing. However, Velcro® is able to accumulate hair, dust, and fiber in the hooks and reduce the efficiency of the fastener. Further, Velcro's® hooks are able to easily grab onto woven items such as a sweater and are able to cause damage to the item upon removal.